1. Field of the Invention
The present invention relates to a document feeder for feeding a document, and an image reader in which such a document feeder is provided.
2. Related Arts
Recently, image readers have come to be widely employed for reading images from documents. For many of these image readers, document feeders are provided for automatically feeding documents to the image readers.
To efficiently read a document, some image readers that are mainly for professional use employ document feeders which can mount a great number (e.g., several hundred or a thousand sheets) of documents at one time on hopper tables.
Generally, a document feeder is required to satisfy the following conditions.
First, a feeding angle at which a document is fed to a feeding roller in an image reader (the same angle as the angle of a hopper table) must not exceed a permissible range for an ideal angle, regardless of the number of documents remaining on the hopper table. This is because if the spread of the feeding angle exceeds the permissible range, paper feeding errors, such as the jamming of documents or that a plurality of sheets are fed at one time, may occur.
Second, the external dimension of the document feeder is small enough to save an installation space.
Third, a power required for raising and lowering a document (on a hopper table) is as small as possible in order to save the electric power consumption and to reduce the size of a drive source.
Conventional document feeders can generally be classified into two types: a parallel lift type and a swing type. In the parallel lift type document feeder, documents are stacked flat in a box-shaped or frame-shaped magazine and are raised and lowered with the magazine in the horizontal state by the action of lifting means, so that the topmost sheet of the documents is fed to the level of a feeding roller.
This structure, however, has the following drawback. A distributed load imposed by the documents affects the magazine. When a large number of documents are mounted, such a distributed load is considerable.
For the parallel lift feeder, two reactive forces that support two ends of the documents against the distributed load must be coped with by the power of the drive source. Further, not only the document but also the magazine must be raised or lowered. Therefore, a drive source that can generate a considerably great power is required. As a result, the size of the document feeder is increased, and a drive source having a large capacity is required, so that the consumption of electric power is increased.
In the swing type document feeder, an end of the hopper table is pivotally mounted on a specific shaft (i.e., is supported rotatably between a convex portion and a concave portion), so that the hopper table can be swung around the rod in accordance with the number of documents mounted on the hopper table.
With this structure, even when a large number of documents are mounted on the hopper table, among the reactive forces related to the distributed load imposed by the documents, the reactive force on the shaft side is coped with by the static reactive force encountered at the rod, and no load is placed on a drive source. Therefore, the required power of the drive source is smaller than that of the parallel lift feeder, and then this is preferable.
However, in order to restrict the angle of the hopper table for this structure within the permissible range, the length of the hopper table (the radius measured from the shaft) must be extended, so that the size of the document feeder is increased. If the length of the hopper table is reduced, the angle through which the hopper table swings must be enlarged so as to handle a large number of documents. As a result, the swing angle of the hopper will exceed the permissible range. Therefore, paper feed errors will occur frequently, and then the image reading efficiency will be drastically reduced.